Many types of printing systems include one or more printheads that have arrays of dot forming elements that are controlled to make marks of particular sizes, colors, or densities in particular locations on the print media in order to print the desired image. In some types of printing systems the array(s) of dot forming elements extends across the width of the page, and the image can be printed one line at a time. However, the cost of a printhead that includes a page-width array of marking elements is too high for some types of printing applications, so a carriage printing architecture is used.
In a carriage printing system (whether for desktop printers, large area plotters, etc.) the printhead or printheads are mounted on a carriage that is moved past the recording medium in a carriage scan direction as the dot forming elements are actuated to make a swath of dots. At the end of the swath, the carriage is stopped, printing is temporarily halted and the recording medium is advanced. Then another swath is printed, so that the image is formed swath by swath. In a carriage printer, the dot forming element arrays are typically disposed along an array direction that is substantially parallel to the media advance direction, and substantially perpendicular to the carriage scan direction. The length of the dot forming element array determines the maximum swath height that can be used to print an image.
Many carriage printing systems include multipass print modes in which the dots in a given region of the recording medium are formed in a plurality of printing passes. In multipass printing, responsibility for printing each raster line of the image is shared between a plurality of dot forming elements. In this way the nonuniform marking behavior of dot forming elements can be disguised in order to provide improved image quality. For an inkjet printer, multipass printing can provide time for improving the uniformity of ink-media interactions by controlling the pattern of dots that can be printed within one pass, thereby reducing coalescence (i.e. flowing together of ink drops on the surface of the page before they soak into the page). Multipass printing can also enable multitone printing in which multiple dots are printed in the same pixel locations.
For a color printer it can be advantageous to have arrays of dot forming elements for plurality of different colors resident on a single printing device within a single printhead. Printing devices that are fabricated using typical manufacturing technologies, including those used in the semiconductor industry, have dot forming elements on a single device that are very uniform in characteristics and well aligned to one another. This is advantageous because print quality is improved if the resulting dots are well-aligned to one another, and the printer can operate more reliably if operating conditions (including voltage and pulsewidth for forming dots) can be selected such that they are optimal or nearly optimal for all of the dot forming elements.
Conventional multicolor printing devices include a plurality of dot forming arrays for forming a respective plurality of different colored dots, but the spacing of dot forming elements within each array on such conventional devices is substantially the same for each array. As a result, the amount of space required on a printing device for M different arrays of dot forming elements (plus the electronics, leads, and colorant supply pathways for each array) is substantially equal to M times the space required for a single array. The size of the printing device is a significant contributor to the cost of a printhead. Because many printing devices are typically made simultaneously using wafer processing, the larger the printing device is, the fewer that can be fit on a wafer, and therefore the higher the cost of the printing device. In addition, a defect is more likely to occur on a larger device than on a smaller device, leading to a lower yield and therefore a higher cost for larger printing devices. Of course, it is possible to reduce the size of a conventional printing device by shortening the length of the arrays of dot forming elements. However, this would result in more printing passes being required in order to print an image, and therefore a longer time required to print an image. What is needed is a multicolor printing device having a smaller size than a conventional multicolor printing device, but without a substantial penalty in printing throughput.